Along with the increasing concern for preserving the world environment comes a call from the adhesives industry for adhesive systems which avoid the use of highly volatile and environmentally detrimental solvents. One possible reply is the utilization of aqueous adhesive compositions wherein the polymeric materials contained therein are emulsion or otherwise polymerized in the presence of water. Traditional surfactants utilized in preparing emulsion-polymerized polymers include anionic and nonionic surfactants such as alkali alkyl sulfates, ethoxylated aryl sulfonic acid salts and ethoxylated alkyl aryl derivatives. A latex (polymer in water dispersion) prepared utilizing such traditional surfactants necessarily contains surfactant molecules in the latex. The presence of these surfactant molecules can have a deleterious effect (i.e., result in a "surfactant penalty") upon adhesive or other similar compositions containing the surfactants, especially when the compositions are exposed to high temperature or other severe environments. This surfactant penalty is particularly evident in adhesive compositions since the surfactant molecules directly interfere with the ability of the adhesive molecules to interact with a surface being bonded. Furthermore, emulsions prepared with conventional type surfactants are relatively unstable and will readily undergo coagulation in the presence of solvents or divalent ions which may be present as impurities. Conventional emulsions are also easily destroyed by freezing temperatures.
An aqueous emulsion polymerization process is disclosed in British Patent No. 1,166,810. The process involves the emulsion polymerization of chloroprene, alone or with copolymerizable comonomers, in an emulsifying composition which contains potassium salts of a rosin acid, a formaldehyde condensate of a naphthalene sulfonic acid and a polymerized unsaturated fatty acid. It has also previously been proposed to utilize polyvinyl alcohol in an emulsion polymerization process. For example, British Patent No. 1,469,993 describes the formation of polychloroprene latices by polymerizing an aqueous suspension of chloroprene monomers in the presence of polyvinyl alcohol and a dialkyl xanthogen disulfide or an alkyl mercaptan.
U.S. Pat. No. 3,920,600 describes a neoprene latex used in pressure sensitive adhesive coatings which contains an alkylated melamine-aldehyde resin, in combination with an aqueous dispersion of a copolymer prepared by polymerizing an aqueous suspension of chloroprene monomers and .alpha.,.beta.-unsaturated carboxylic acid monomers in the presence of polyvinyl alcohol and a dialkyl xanthogen disulfide or an alkyl mercaptan. U.S. Pat. No. 4,128,514 discloses a process for preparing latex compositions by polymerizing an aqueous suspension of chloroprene and .alpha.,.beta.-unsaturated carboxylic acid monomers in the presence of hydroxyalkyl cellulose, polyvinyl alcohol, and an alkyl mercaptan, or a dialkyl xanthogen disulfide.
A need exists for a latex composition which would exhibit substantial bonding activity with a minimal or non-existent surfactant penalty and which would exhibit stability in the presence of solvents, divalent ions, and freezing temperatures.